A package for storing and/or transporting radioactive materials generally comprises, as outer containment, a packaging having a lateral body, a bottom and a cover. These parts of the packaging define a cavity for housing an assembly containing radioactive materials, for example a basket housing nuclear fuel assemblies or waste canisters.
The safety demonstration of the packaging loaded with the assembly is based in particular on regulatory drop tests. In the case of a drop from a height of 9 meters in the direction of the axis of the packaging passing through the bottom and the cover thereof, on the head dampening overcap covering the cover of the packaging, the total weight of the assembly containing the radioactive materials presses on said same cover during impact on the ground. During this drop, known as “axial drop”, very considerable stresses are generated in the system of closing the cover on the lateral body of the packaging. In particular, the fixing screws are highly stressed, and, under certain conditions, the moveable assembly in the cavity of the packaging can impact the cover with a particularly damaging effect on the closing system.
In order to assure the leak tightness of the packaging after the axial drop, it may thus prove to be necessary to limit the stresses transmitted by the assembly on the cover, by means of an impact dampening system placed on the inner surface of the cover.
Generally, such a system comprises at least one deformable dampening device and a loading device associated with said dampening device. The two aforementioned devices are respectively fixed on the assembly containing the radioactive materials and on the inner surface of the cover. For example, in the case where said assembly integrates canisters, each canister placed opposite the cover integrates a deformable dampening device whereas opposite the latter is placed a loading device in the form of a chock, fixed on the inner surface of the cover.
To obtain optimal crushing of the dampening device, and thus dissipate as best as possible the mechanical energy through the deformation of the dampening device, precise alignment is required between the latter and its associated loading device.
Yet, although the cover of the packaging on which one of the devices is fixed is generally positioned precisely on the lateral body of the packaging, there exists on the other hand often significant lateral play between the assembly and the internal wall of the lateral body, particularly to enable the loading thereof in the cavity, thus creating an alignment defect.
In order to assure the best alignment possible between these two devices, it may thus be envisaged to considerably limit the lateral plays defining the position of the canisters with respect to the loading chocks. In other words, this leads in particular to reducing the play between the assembly containing the radioactive materials and the inner surface of the packaging lateral body.
The reduction of these lateral plays poses not only operational constraints during the loading of the canisters in the housings of the basket and during the loading of said basket in the cavity of the packaging, but also induces manufacturing constraints due to the necessity of managing low tolerances.
Obviously, these drawbacks presented within the scope of an assembly integrating canisters are encountered whatever the nature of said assembly, in other words whatever the shape of the devices containing the radioactive materials.